High-temperature ovens, which are typically called reactors, can be used to create structures of very fine dimensions, such as, for example, integrated circuits on semiconductor wafers or other substrates. The one or more substrates, such as, for example silicon wafers, are placed on a wafer support inside a reaction chamber of the reactor. Inside the reaction chamber, both the wafer and the support (also called a susceptor) are heated to a desired temperature. In a typical wafer treatment step, reactant gases are passed over the heated wafer, causing the chemical vapor deposition (CVD) of a thin layer of the reactant material on the wafer. If the deposited layer has the same crystallographic structure as the underlying silicon wafer, it is called an epitaxial layer. This layer is also sometimes called a monocrystalline layer because it has only one crystal structure.
“Cold wall” reaction chambers are a type of reaction chamber that are desirably made of quartz (vitreous silica) or other similar materials which are substantially transparent to the radiant energy used for heating the chamber. Quartz is also desirable because it can withstand very high temperatures, and because it is relatively inert (i.e., it does not react with various processing gases typically used in semiconductor processing). Quartz is also typically used to build a number of other reactor components, comprising spiders, which are used to support the susceptors, and stands, which are used to support temperature compensation rings around the periphery of the susceptors. Due to its material characteristics, quartz components are also advantageous for other types of reactors that do not use radiant heating systems.
To ensure the high quality of the resulting layers on the wafers, various process parameters must be carefully controlled. One such critical parameter is the temperature of the reactor and the wafer during each treatment step of the processing. During CVD, for example, the deposition gases react at particular temperatures and deposit on the wafer. For silicon deposition, for example, deposition temperatures can affect the crystal structure of the resultant layers, from amorphous at low temperatures to polycrystalline at intermediate temperatures, to epitaxial (single crystal) at high temperatures. If the temperature varies across the surface of the wafer, uneven deposition rates can result at different points across the wafer, leading to non-uniform thicknesses. Accordingly, it is important that the wafer temperature be stable and uniform at the desired temperature both before the treatment begins and during deposition. Similarly, non-uniformity or instability of temperatures across a wafer during other thermal treatments can affect the uniformity of resulting structures. Other processes for which temperature control can be critical include oxidation, nitridation, dopant diffusion, sputter depositions, photolithography, dry etching, plasma processes, and high temperature anneals.
Typically, semiconductor reactors are operated at relatively high temperatures. The reactor can be frequently cycled up and down from these relatively high temperatures to relatively cold temperatures. Thermocouples are often used to monitor temperatures within the reactor. However, because of the corrosive environments present in the reactor, the thermocouple is typically surrounded by a protective sheath. For example, the thermocouple is coaxially inserted into the protective sheath such that the heat-sensing junction of the thermocouple is placed adjacent to the bottom of the protective sheath. Accordingly, the thermocouple senses the temperature of the reactor through the protective sheath. Such sheaths should be made of a material that withstands high temperatures and thermal cycling as well as the corrosive processing environment. Further, the sheath material should have good thermal conductivity, whereby the sheathed thermocouple will rapidly react to temperature fluctuations. For semiconductor processing applications, the protective sheath is desirably chemically inert and of a suitable chemical purity to avoid contaminating the wafer during processing.
The thermocouples used to measure temperature in CVD reactors are typically protected with quartz sheaths. The inventors have found that, while these quartz sheaths are useful in protecting the thermocouple during wafer processing, in corrosive environments frequent thermal cycling of the quartz sheath to temperatures in excess of 1000° C. can cause devitrification of the quartz sheath. Even in non-corrosive environments, frequent thermal cycling of the quartz sheath to temperatures in excess of approximately 1250° C. can cause devitrification. Some processes, like epitaxy, typically occur at temperatures of 1150° C. or higher. Devitrification is a second order phase transition of amorphous quartz into cristobalite. Devitrification begins at naturally occurring nucleation sites in the amorphous quartz. This phase transition results in a twenty percent density change causing stresses to build up in the cristobalite. When the quartz sheath is allowed to cool down to approximately 275° C. or below, the crystallographic inversion temperature range, the cristobalite cracks. This cracking ultimately causes the sheath to lose its protective function, leading to subsequent failure of the thermocouple, necessitating its replacement.
The need to replace thermocouples, and various other chamber components subject to devitrification, naturally results in downtime for the reactor and significant costs for replacement components. In addition, there is significant time and expense in returning the reactor to the operating conditions necessary to produce the desired film properties on the wafers being coated. Replacing thermocouples and other components requires an intrusion into the chamber which can result in undesirable particle generation. The cristobalite transition, and resultant cracking, occurs most frequently at the tip of the thermocouple sheath where it contacts, or is in close proximity to, the hot susceptor. However, in addition to thermocouple sheaths, other quartz reactor components are potentially subject to the same problems of devitrification. Although the problem of devitrification of quartz into cristobalite has been described herein above, any family of amorphous glass is subject to undesirable devitrification.
Exposing quartz to acidic environments, in combination with high temperatures, exacerbates its devitrification. Although cristobalite can form at temperatures at or above 1150° C. in the absence of an acidic environment, the rate of cristobalite formation in an acidic environment is much more rapid. Many CVD processes, e.g., etching, are performed in acidic environment. Reactor cleaning procedures also introduce acid into the chamber. Generally, in CVD reactors, the reactant material not only deposits on the substrate, as is desired, but some material is also deposited on the reactor walls and other components within the reactor. Periodically, in order to maintain a repeatable process, the reactor has to be cleaned. Reactor cleaning typically occurs by heating the wafer support, reactor walls and other reactor components to a suitably high temperature and admitting a flow of a halogen containing gas, for example HCl. Other typical cleaning gases include Cl2, NF3, ClF3, or mixtures thereof.
There is a need to significantly extend the life of quartz and other vitreous materials used as thermocouple sheaths and for other components within a CVD chamber. Known methods of protecting thermocouples are prohibitively expensive, cannot be used to protect complex components because of fabrication constraints, are a source of contaminants, or are otherwise incompatible with the high temperatures and acidic conditions found in CVD reactors. Therefore, there exists a need to protect quartz and other vitreous materials in an economical manner and without negatively affecting the beneficial properties of these materials.
In satisfaction of this need, the preferred embodiments of the invention provide for a chemical vapor deposition reactor for the processing of semiconductor substrates, wherein the lifetimes of some internal reactor components made of vitreous materials are extended by coating them with a barrier material layer selected for this purpose.
In one arrangement of the invention, a reaction chamber is in the form of a horizontally oriented quartz tube divided into an upper region and a lower region by a front divider plate, a susceptor surrounded by a temperature compensation or slip ring, and a rear divider plate. Mounted adjacent to the susceptor are one or more thermocouples each having a sheath made of a vitreous material which is coated with a barrier material layer which is more durable than vitreous material itself. When coated with the barrier material layer, the thermocouple sheath does not devitrify upon high temperature cycling and, thus, the life of the thermocouple sheath is greatly extended over that of previously used uncoated sheaths. The barrier material layer is especially useful in acidic environments.
In accordance with another arrangement, protective barrier layers are provided on various parts of the chamber to protect quartz reactor components from devitrification. A protective barrier is provided over a quartz sheath covering a thermocouple, thereby protecting the quartz from the processing gases. Protective barrier layers may also be used to cover, either partially or fully, other quartz components, such as the quartz spider supporting the susceptor or the quartz stand supporting the slip ring. It is expected that protecting quartz sheathed thermocouples and quartz components with barrier coatings will significantly increase their lifetime.
In accordance with another aspect of the invention, a method is provided for forming barrier material layers on vitreous materials. Advantageously, the method includes depositing a barrier material selected for this purpose in such a manner that the barrier layer is thin and has good adherence to the underlying vitreous material, resulting in a layer with reasonable thermal conductivity.
All of these embodiments are intended to be within the scope of the invention herein disclosed. These and other embodiments of the present invention will become readily apparent to those skilled in the art from the following detailed description of the preferred embodiments having reference to the attached figures, the invention not being limited to any particular preferred embodiment(s) disclosed.